“Active Mass Absorber” at a 4×4 Transmition System 2003-01-3682
The extensive use of rotative machines in the diverse branches of the modern world has made the rising undesirable mechanical and acoustic vibration levels to be a problem of special importance for the machines normal operation as for the communities that are each time more affected by the problem. It makes the study of vibration and acoustic phenomena also to be even more important and the applications of its concepts more sophisticated.
Several are the concepts used for decreasing vibration levels, like common dampers, hydraulic dampers, active dampers, natural frequencies changes and others. The choice of use of one or another depends greatly on the engineering possibilities (weight, energy, physical space, other components functional interference, vibration levels, etc.) as well as the cost of implementation of each one. As a general rule, normal dumpers is usually the best method to decrease vibration levels, because of the smaller prices and not often collateral undesirable effects.
In this work is presented a problem of vibration of the powertrain system of a 4×4 car that could not be easily solved by the use of normal dumpers and therefore another vibration-decreasing concept has been used: the “Active Mass Absorber”.
“Dynamic Absorber” is a concept based on a phenomenon related to one specific vibration absorption by one auxiliary vibrating system attached to the main system, which the vibration level is wanted to be decreased. It works as it was draining the vibration movement of one specific frequency, so the main system vibration level decreases and the attached system (active mass damper) vibrates.
In a normal car there are several exciting vibration and acoustic sources, such as the engine, tires, road roughness, air intake and exhaust system. The active mass absorber concept, which is designed for one only frequency absorption, is not an effective approach for the general vibration and acoustic problem solution.
In the present study, however, the second order engine vibration was detected to be transmitted to the rear part of the car by the driveshaft and resonating in the back seats area. This resonation caused high sound pressure levels inside the vehicle and could not be decreased by normal dampers between the rear differential and the vehicle body.
The solution proposed to this problem was attaching an active mass damper to the rear differential that absorbs part of the vibration energy before it flows to the car body.